Turbines generally include a rotor comprised of a plurality of rotor turbine wheels, each of which mounts a plurality of circumferentially-arranged buckets. Each bucket includes an airfoil, a platform, a shank and a dovetail, the dovetail being received in mating dovetail slot in the turbine wheel. The airfoils project into a hot gas path downstream of the turbine combustors and convert kinetic energy into rotational, mechanical energy.
Often, a protective coating is applied to the turbine wheel for various purposes. For example, the turbine wheel can be instrumented for component and developmental testing (CDT). In CDT, sensors or instruments are attached to the turbine wheel—often by resistance welding the sensors to the turbine wheel. Rather than resistance welding the sensors directly to the turbine wheel itself, a nickel-chromium (NiCr) coating can be applied to the turbine wheel using a plasma spray for example. The sensors then can be welded to the protective coating. In this way, the turbine wheel can be instrumented without inducing or creating stress risers into the base/parent material of the turbine wheel.
However, it is necessary to prevent the dovetail slots from being coated. The slots, which are critical to the usable life of the turbine wheel, are machined to a precisely shaped profile and surface finish. Complementarily shaped dovetails (also precisely machined) of the buckets are mated with the slots for assembly of the turbine. Due in large part to the precise machining of the dovetails and slots, the usable life of the turbine would be compromised if the slots are coated. The coating can be removed, but the removal process generally requires an abrasive device, which disturbs the surface finish. Any disturbance of the dovetail surface can decrease the usable life of the turbine wheel and negate any applied metal treatments such as shotpeen.
Prior attempts to prevent the slots from being coated included using high temperature adhesive tapes to mask off the dovetail slots and other critical areas. This is a labor intensive and a time consuming process. Also, the tapes can create sharp edges that can result in coating chipping and flaking which requires extensive detail and blending post processing to remove such defects. In addition, the plasma spray is applied at high pressures, such as at 90 PSI. This can cause the tape to lift allowing overspray to come in contact with the dovetail surface.
Thus, it is desirable to provide a method and a device to apply protective coating with a greater control of pattern definition, coating surface finish, and to eliminate or vastly reduce incidences of process damage and the necessary re-work that follows such incidences.